U.S. patent application number 10/382374 was filed with the patent office on 2003-09-11 for micro-electromechanically tunable vertical cavity photonic device and a method of fabrication thereof.
Invention is credited to Iakovlev, Vladimir, Kapon, Elyahou, Rudra, Alok, Sirbu, Alexei, Suruceanu, Grigore.
Application Number | 20030169786 10/382374 |
Document ID | / |
Family ID | 25200850 |
Filed Date | 2003-09-11 |
United States Patent
Application |
20030169786 |
Kind Code |
A1 |
Kapon, Elyahou ; et
al. |
September 11, 2003 |
Micro-electromechanically tunable vertical cavity photonic device
and a method of fabrication thereof
Abstract
A tunable Fabry-Perot vertical cavity photonic device and a
method of its fabrication are presented. The device comprises top
and bottom semiconductor DBR stacks anid a tunable air-gap cavity
therebetween. The air-gap cavity is formed within a recess in a
spacer above the bottom DBR stack. The top DBR stack is carried by
a supporting structure in a region thereof located above a central
region of the recess, while a region of the supporting structure
above the recess and outside the DBR stack presents a membrane
deflectable by the application of a tuning voltage to the device
contacts.
Inventors: |
Kapon, Elyahou; (Lausanne,
CH) ; Iakovlev, Vladimir; (Ecublens, CH) ;
Sirbu, Alexei; (Ecublens, CH) ; Rudra, Alok;
(Blonay, CH) ; Suruceanu, Grigore; (Ecublens,
CH) |
Correspondence
Address: |
MOETTELI & ASSOCIES SARL
CASE POSTALE 486
AVE DE FRONTENEX 6
GEVEVA 12
CH-1211
CH
|
Family ID: |
25200850 |
Appl. No.: |
10/382374 |
Filed: |
March 6, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10382374 |
Mar 6, 2003 |
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09809236 |
Mar 15, 2001 |
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6546029 |
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10382374 |
Mar 6, 2003 |
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PCT/IB02/00682 |
Mar 8, 2002 |
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Current U.S.
Class: |
372/20 ;
372/45.01; 372/96; 438/22; 438/31; 438/47 |
Current CPC
Class: |
H01S 5/18366 20130101;
H01S 5/041 20130101; H01S 5/18341 20130101; H01S 5/34306 20130101;
B82Y 20/00 20130101; H01S 5/1838 20130101 |
Class at
Publication: |
372/20 ; 438/22;
372/45; 372/96; 438/31; 438/47 |
International
Class: |
H01S 005/00; H01S
003/10; H01L 021/00; H01S 003/08 |
Claims
What is claimed is:
1. A method of fabrication of a Fabry-Perot tunable vertical cavity
device comprising first and second distributed Bragg reflector
(DBR) stacks with a tunable air-gap cavity therebetween, the method
comprising the steps of: (a) forming a spacer above the first DBR
stack, wherein said spacer has a structured surface formed by a
recess presenting a location for the tunable air-gap cavity; (b)
providing a second DBR structure and coupling it to the structured
surface of the spacer so as to completely cover said recess by said
second DBR structure, thus forming the air-gap cavity between the
first DBR stack and the second DBR structure, and; (c) selectively
applying material removal to the second DBR structure to form a
mesa, presenting the second DBR stack, above a region of said
recess, and to form a membrane above said recess outside said
second DBR stack.
2. The method according to claim 1, comprising forming electrical
contacts of the device on regions of the upper surface of the first
DBR stack and on regions of the upper surface of the second DBR
structure outside said mesa, thereby enabling deflection of said
membrane by application of a tuning voltage to the electrical
contacts of the device.
3. The method according to claim 1, wherein the second DBR
structure is coupled to the structured surface of the spacer by
bonding.
4. The method according to claim 1, wherein said second DBR
structure comprises a supporting structure carrying a layer
structure of the second DBR stack.
5. The method according to claim 4, wherein said material removal
comprises etching the layers of the second DBR structure until the
supporting structure is reached.
6. The method according to claim 4, wherein said second DBR
structure comprises an intermediate layer presenting an interface
between the supporting structure and the layer structure of the
second DBR stack.
7. The method according to claim 6, wherein said material removal
comprises etching the layers of the second DBR structure until said
intermediate layer is reached, such layer serving as an etch stop
layer.
8. The method according to claim 1, wherein said mesa is centered
about a vertical axis passing through the center of said
recess.
9. The method according to claim 1, wherein the recess is formed by
etching all the layers of the spacer.
10. The method according to claim 1, wherein the depth of the
recess defining the thickness of the air-gap cavity is in the range
of 0.5-1.5 .mu.m.
11. The method according to claim 4, wherein the lateral
continuation of the supporting structure within a region thereof
outside the region below the mesa forms the membrane completely
covering the recess.
12. The method according to claim 11, wherein the thickness of the
membrane is about 0.5-1.5 micron.
13. The method according to claim 4, wherein the second DBR stack
is coupled to the structure surface of the spacer by applying a
wafer fusion between the surface of the supporting structure of the
second DBR structure and the structured surface of the spacer.
14. The method according to claim 1, comprising formation of a mesa
on the bottom of said recess.
15. The method according to claim 14, wherein said mesa on the
bottom of the recess is centered about the central vertical axis
passing through the center of the recess.
16. The method according to claim 14, wherein said mesa on the
bottom of the recess has a lateral size and a height of less than
10 and less than {fraction (1/30)}, respectively, of a certain
wavelength selected as an operational wavelength of the device.
17. The method according to claim 1, comprising formation of an
active cavity material between the first DBR stack and the
spacer.
18. The method according to claim 17, wherein the formation of the
active cavity material comprises the steps of growing a
multiquantum well layer stack sandwiched between two cladding
layers.
19. The method according to claim 17, wherein said active cavity
material is fused to the surface of the first DBR stack.
20. The method according to claim 1, wherein the spacer is a stack
formed by layers having the same thickness and composition values
as in the first DBR stack.
21. The method according to claim 20, wherein the layers in the
spacer have alternating n-type and p-type doping.
22. The method according to claim 1, wherein the second DBR stack
comprises pairs of Al.sub.xGa.sub.1-xAs layers with different
values of x.
23. The method according to claim 4, wherein the supporting
structure comprises pairs of Al.sub.xGa.sub.1-xAs layers with
different values of x.
24. The method according to claim 23, wherein the supporting
structure comprises the same pairs of Al.sub.xGa.sub.1-xAs layers
as the second DBR stack.
25. The method according to claim 1, wherein each of the first and
second DBR stacks comprises pairs of Al.sub.xGa.sub.1-xAs layers
with different values of x.
26. The method according to claim 23, wherein the second DBR
structure comprises an etch stop layer presenting an interface
between the second DBR stack and the supporting structure.
27. The method according to claim 23, wherein said etch stop layer
is an InGaP layer.
28. The method according to claim 1, wherein the first DBR stack
comprises 30 pairs of AlGaAs/GaAs n-type layers grown on an n-type
GaAs substrate.
29. The method according to claim 28, wherein said spacer is a
stack having six pairs of AlGaAs/GaAs layers with alternating
n-type and p-type doping.
30. The method according to claim 29, wherein said recess is formed
in the spacer by reactive plasma dry etching in
Cl.sub.2--CH.sub.4--Ar and selective chemical etching in a
HF--H.sub.2O solution.
31. The method according to claim 30, wherein the etching is
stopped, when reaching the top GaAs layer of the first AlGaAs/GaAs
DBR stack.
32. The method according to claim 1, wherein the second DBR
structure contains a GaAs substrate carrying a layer structure of
the second DBR stack, and a supporting structure on top of said
layer structure.
33. The method according to claim 32, wherein the second DBR stack
is coupled to the structured surface of the spacer by applying a
wafer fusion between the surface of the supporting structure of the
second DBR structure and the structured surface of the spacer.
34. The method according to claim 33, wherein the fusion is
performed at 650.degree. C. by applying a pressure of 2 bar to the
fused interface.
35. The method according to claim 34, wherein said selective
material removal comprises selectively etching the GaAs-substrate
in a H.sub.2O.sub.2--NH.sub.3OH solution till reaching the first
AlGaAs layer of the second DBR layer structure, said first layer
acting as an etch-stop layer.
36. The method according to claim 35, comprising selectively
etching said etch-stop layer in a HF--H.sub.2O solution.
37. The method according to claim 32, wherein said selective
material removal comprising etching the mesa in the second DBR
structure by dry etching in Cl.sub.2--CH.sub.4--Ar and selective
chemical etching in a HF--H.sub.2O solution.
38. The method according to claim 37, wherein said etching of mesa
continues until an etch stop-layer in the second DBR structure is
reached, thus presenting an interface between a support structure
for supporting the DBR stack in the second DBR structure.
39. The method according to claim 14, comprising formation of an
active cavity material between the first DBR stack and the
spacer.
40. The method according to claim 39, wherein the formation of the
active cavity material comprises the steps of growing a
multiquantum well layer stack sandwiched between two cladding
layers.
41. The method according to claim 39, wherein said active cavity
material is fused to the surface of the first DBR stack.
42. The method according to claim 39, wherein said first DBR stack
is AlGaAs/GaAs, and said active cavity material comprises a
multiquantum well InGaAsP/InGaAs layer stack sandwiched between two
InP cladding layers.
43. The method according to claim 42, wherein said spacer comprises
a InP layer with alternating p-n-p-n doping sandwiched between 2
InGaAsP etch-stop layers.
44. The method according to claim 39, wherein said mesa on the
bottom of the recess is centered about the central vertical axis
passing through the center of the recess.
45. The method according to claim 44, wherein said mesa on the
bottom of the recess has a lateral size and a height of less than
10 and less than {fraction (1/30)}, respectively, of a certain
wavelength selected as an operational wavelength of the device.
46. A tunable Fabry-Perot vertical cavity device fabricated by the
method of claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of
PCT/IB02/00682, filed on Mar. 8, 2002 and U.S. Ser. No 09/809,236,
filed on Mar. 15, 2001, all of the same title, the applications
having common inventors, and the contents of which being
incorporated herein by reference thereto. Priority under 35 U.S.C.
.sctn.119 is claimed to these prior applications.
FIELD OF THE INVENTION
[0002] The present invention is generally in the field of
semiconductor optoelectronic devices, and relates to
micro-electromechanically tunable vertical cavity photonic devices,
such as filters and lasers, and a method of their fabrication.
BACKGROUND OF THE INVENTION
[0003] Tunable optical filters and tunable Vertical Cavity Surface
Emitting Lasers (VCSELs) based on micro-electromechanical
Fabry-Perot filter technology have recently generated considerable
interest in the art. This is due to the fact that these devices
present low cost alternatives to standard tunable filters, lasers
and photodetectors which normally are high cost components, and for
this reason, cannot be used in emerging wavelength
division-multiplexing (WDM) local area networks systems which are
very cost sensitive.
[0004] A micro-electromechanical tunable vertical cavity device
operating in a specific wavelength range represents a Fabry-Perot
cavity formed between two distributed Bragg reflectors (DBRs) that
have high reflectivity values in this specific wavelength range.
The Fabry-Perot cavity incorporates a tunable air gap cavity with a
thickness of about a number of half-wavelengths. Normally, the top
DBR is suspended on a micro-mechanical cantilever (or a number of
micro-beams) above the air gap and can be deflected by changing the
electric field in the air-gap cavity. This changes the wavelength
of resonance of the Fabry-Perot cavity. The higher the reflectivity
of the DBRs, the narrower the linewidth of the transmission
wavelength in a tunable filter. Lower threshold gain and higher
selectivity are achieved, respectively, in tunable VCSELs and
resonant photodetectors.
[0005] Semiconductor based DBRs, which have low optical absorption,
good thermal conductivity and reflectivity values in excess of
99.5%, are widely used in the art for the fabrication of different
types of micro-electromechanically tunable vertical cavity
devices.
[0006] U.S. Pat. No. 5,771,253* discloses a tunable VCSEL device
based on the micro-electromechanical Fabry-Perot filter technology
which comprises an electrically deflectable cantilever, a top and
bottom DBR and a multiquantum well (MQW) region. The MQW well
region is situated between a bottom DBR and a top reflector
consisting of a partial DBR situated on top of the MQW, an air-gap
and a moveable DBR situated on the cantilever. An oxide layer is
situated in the partial DBR to provide lateral electrical and
optical confinement in the active region.
[0007] The article "Widely and continuously tunable micromachined
resonator cavity detector with wavelength tracking"* M. S. Wu, E.
S. Vail, G. S. Li, W. Yuen and C. J. Chang-Hasnain, IEEE Photon.
Technol. Lett., 8, (1996), No 1, pp. 98-100, discloses a tunable
photodetector based on the micro-electromechanical Fabry-Perot
filter technology which comprises an electrically deflectable
cantilever, top and bottom DBR stacks and a photodetector region
situated between top and bottom DBRs.
[0008] The article "GaAs Micromachined Widely Tunable Fabry-Perot
Filters",* E. C. Vail et al., Electronics Letters Online, Vol. 31,
No. 3, 1995, pp. 228-229, discloses a process of fabrication of a
tunable optical filter of the kind specified. First, a monolithic
structure is formed consisting of top and bottom DBRs separated by
a sacrificial layer. Then, the top DBR is structured by etching it
completely in unmasked regions until reaching the sacrificial
layer. This process is followed by selectively etching the
sacrificial layer in unmasked regions and under the top DBR and
supporting cantilever. This results in that the top DBR is
suspended above the bottom DBR and in an air gap between the top
and bottom DBRs having a thickness approximately equal to the
thickness of the sacrificial layer. The remaining part of the
sacrificial layer fixes the cantilever at its base.
[0009] All cantilever-based devices have a complex fabrication
process and are mechanically unstable, which results in a low
fabrication yield. These devices are also difficult to optimize: if
the cantilever is longer than 100 .mu.m, the mechanical instability
drastically increases. In case of shorter cantilevers, the
flexibility is reduced, resulting in the necessity to decrease
their thickness. This results in the reduction of the number of
pairs in the top DBR stack, and consequently, in inferior device
parameters.
[0010] A different technique of fabrication of an electrically
tunable optical filter is disclosed in U.S. Pat. No. 5,739,945 and
in the article "Widely Tunable Fabry-Perot Filter Using
Ga(Al)As--AlO.sub.x Deformable Mirrors"*, P. Tayebati et al., IEEE
Photonics Technology Letters, Vol. 10, No. 3, 1998, pp. 394-396.
According to this technique, the low index AlGaAs layers of a
conventional mirror stack consisting of GaAs and AlGaAs layers is
substituted with oxidized AlGaAs layers or air gaps. Although this
technique provides quite good results, i.e., the tuning range of 70
nm around 1.5 .mu.m was obtained by applying a voltage of 50V, the
fabrication process is very complex and the device structure
obtained with this technique is even more mechanically unstable
than standard cantilever-type devices.
SUMMARY OF THE INVENTION
[0011] There is accordingly a need in the art to improve
micro-electromechanically tunable vertical cavity photonic devices
by providing a novel device structure and fabrication method.
[0012] The main idea of the present invention consists in replacing
cantilevers and beams which support top DBRs in the prior art
devices of the kind specified by a membrane, which completely
covers an air-gap cavity and carries the top DBR stack, which is
situated in the center of the membrane. The air-gap is incorporated
in an etched-through recess in a spacer which is blocking the
current flow when applying a voltage to the device contacts to
deflect the membrane. Membrane deflection results in tuning the
air-gap cavity and, as a consequence, the resonance wavelength of
the device.
[0013] The above is implemented in the following manner: First, the
surface of a spacer is structured by etching a recess through it.
Then, a supporting structure, on which a DBR is located, is bonded
to the structured surface of the spacer. This is followed by
etching the DBR till reaching the supporting region, thereby
forming a mesa of the top DBR stack. The mesa is centered around a
vertical axis passing through the center of the recess and has the
lateral dimension less than that of the recess. A region of the
supporting structure outside the top DBR stack (mesa) and above the
recess presents the membrane.
[0014] The membrane is, on the one hand, very flexible (having the
thickness of about 1 .mu.m), and, on the other hand, is continuous
in the lateral direction, and is therefore mechanically stable,
resulting in a high fabrication yield. The top DBR can be made of a
large number of layers without affecting the flexibility of the is
membrane and providing a narrow linewidth of transmitted light. By
forming an island of high refractive index material in the way of
the optical beam inside the optical cavity of the device, the
position of the beam during the tuning process is stabilized.
[0015] Thus, according to one aspect of the present invention,
there is provided a Fabry-Perot tunable vertical cavity device
comprising top and bottom semiconductor DBR stacks separated by a
tunable air-gap cavity and a supporting structure that carries the
top DBR stack, wherein the air-gap cavity is located within a
recess formed in a spacer completely covered by the supporting
structure, the top DBR stack being centered around a vertical axis
passing through the center of said recess and having a lateral
dimension smaller than the lateral dimension of the recess, a
region of the supporting structure above the recess and outside the
top DBR stack presenting a membrane to be deflected by application
of a tuning voltage to electrical contacts of the device.
[0016] According to another aspect of the present invention, there
is provided a method of fabrication of a Fabry-Perot tunable
vertical cavity device comprising top and bottom DBR stacks with a
tunable air-gap cavity therebetween, the method comprising the
steps of:
[0017] (i) forming a spacer above the bottom DBR stack;
[0018] (ii) fabricating an etched-through recess in the spacer,
thereby forming a structured surface of the spacer, said recess
presenting a location for said tunable air-gap cavity;
[0019] (iii) bonding a top DBR wafer including a supporting
structure to the structured surface of the spacer in such a way
that said supporting structure faces said structured surface of the
spacer and completely covers said recess, thus forming the air-gap
cavity, and selectively etching a substrate on which layers of the
top DBR were grown;
[0020] (iv) forming the top DBR stack above a central region of
said recess and a membrane above said recess outside said top DBR
stack, by etching the layers of the top DBR till reaching the
supporting structure so as to define a mesa presenting said top DBR
stack having a lateral dimension smaller than the lateral dimension
of said recess and being centered about a vertical axis passing
through the center of said recess, a region of the supporting
structure above said recess and outside said mesa presenting said
membrane deflectable by application of a tuning voltage to
electrical contacts of the device.
[0021] In order to confine the optical mode of transmitted or
emitted light, a mesa can be formed on the bottom of the recess
being centered around the vertical axis passing through the center
of the recess and having the lateral size of less than 10 and
height of less than {fraction (1/30)} of the device operation
wavelength.
[0022] The spacer region can be placed on top of the bottom DBR, in
which case the device presents a tunable optical filter. In the
case of tunable VCSELs and tunable resonant photodetectors, an
active cavity material is placed between the spacer and the bottom
DBR.
[0023] The top DBR stack may comprise pairs of layers of
Al.sub.xGa.sub.1-xAs with different values of x, and the supporting
structure and the bottom DBR stack may also comprise the same pairs
of layers as in the top DBR stack. The spacer may comprise layers
with alternating n-type and p-type doping. In the case of the
tunable filter, the spacer may comprise the same pairs of layers as
in the bottom DBR with alternating n- and p-type doping. In the
case of tunable VCSELs and tunable resonant photodetectors, the
spacer may comprise layers grown in the same material system as
layers in the active cavity material stack with alternating n- and
p-type doping.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] In order to understand the invention and to see how it may
be carried out in practice, several embodiments will now be
described, by way of non-limiting examples only, with reference to
the accompanying drawings, in which:
[0025] FIG. 1 illustrates an example of a tunable optical filter
device according to the present invention;
[0026] FIG. 2 illustrates the fabrication of the filter device of
FIG. 1;
[0027] FIG. 3 illustrates an example of a tunable VCSEL device
according to the present invention; and
[0028] FIGS. 4 and 5 illustrate the fabrication of the tunable
VCSEL device of FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
[0029] Referring to FIG. 1, there is schematically illustrated a
tunable vertical cavity device, generally designated 10,
constructed according to one embodiment of the present invention.
The device 10 is designed like a Fabry-Perot vertical cavity based
device, having two semiconductor DBRs 12a and 12b, and an air-gap
cavity 14 therebetween, and presents a tunable optical filter. The
air-gap cavity 14 is located within an etched-through recess 16
formed in a spacer 17, which is located on top of the bottom DBR
12b and is completely covered by a supporting structure 18, which
carries the top DBR stack 12a. The top DBR stack 12a is located on
a region 18a of the supporting structure 18 so as to be centered
around a vertical axis passing through the center of the recess 16.
The top DBR stack 12a has a lateral dimension smaller than that of
the recess 16. A region 18b of the supporting structure outside the
region 18a (carrying the top DBR stack 12a) presents a membrane 23
deformable by the application of a tuning voltage to the device
contacts 26.
[0030] In the present example, the bottom DBR 12b comprises 30
pairs of AlGaAs/GaAs n-type layers grown on a n-type GaAs substrate
and having the reflectivity of 99.5% at 1.55 .mu.m. The spacer 17
is a stack of six pairs of AlGaAs/GaAs layers with the same
thickness and composition values as in the bottom DBR stack 12b. In
distinction to the layer structure of the bottom DBR stack, the
layers in the spacer 17 have alternating n-type and p-type doping.
The recess 16 with a lateral dimension of 300.times.300 .mu.m.sup.2
is made by etching all six layers of the spacer 17, such that the
depth of the recess 16 is equal to about 1.5 .mu.m, which defines
the thickness of the air-gap cavity 14, and the bottom surface 20
of the recess 16 coincides with the top of the bottom DBR stack
12b.
[0031] The top DBR stack 12a is a mesa containing 25 pairs of
AlGaAs/GaAs layers, and having the reflectivity of 99.7% and the
lateral dimension of 80.times.80 .mu.m.sup.2. The top DBR stack 12a
is located on the supporting structure 18 (within the region 18a
thereof), which consists of 4 pairs of AlGaAs/GaAs layers with the
same thickness and composition as the layers in the top DBR stack
12a, and terminates with a InGaP etch-stop layer 19. The layer 19
has the thickness of 30 nm and is located at the interface between
the top DBR 12a and the supporting structure 18. The lateral
continuation of the supporting structure 18 within the region 18b
thereof (outside the region 18a) forms the membrane 23 which
completely covers the recess 16.
[0032] The fabrication of the filter device 10 will now be
described with reference to FIG. 2.
[0033] In the first step, the etched-through recess 16 with the
lateral size of 300.times.300 .mu.m.sup.2 is formed in the spacer
17 (consisting of a stack of six pairs of AlGaAs/GaAs layers with
alternating n-type and p-type doping) by reactive plasma dry
etching in Cl.sub.2--CH.sub.4--Ar and selective chemical etching in
a HF--H.sub.2O solution. This procedure allows to precisely stop
the etching, when reaching the top GaAs layer of the bottom
AlGaAs/GaAs DBR stack 12b (grown on a substrate 11), which results
in the recess depth of about 1.5 .mu.m.
[0034] In the second step, a wafer fusion is applied between the
surface of the supporting structure 18 of a top DBR wafer 24 and
the structured surface of the spacer 17. The top DBR wafer 24
contains a DBR 12 (in which the top DBR 12a is then formed) grown
on a GaAs substrate 25, and the supporting structure 18 grown on
top of the DBR 12. Hence, the surface of the supporting structure
18 is fused face to face with the structured surface of the spacer
17 forming a fused interface within a surface region of the spacer
17 outside the recess. The fusion is performed at 650.degree. C. by
applying a pressure of 2 bar to the fused interface. Thereafter,
although not specifically shown here, the GaAs-substrate 25 is
selectively etched in a H.sub.2O.sub.2--NH.sub.3OH solution till
reaching the first AlGaAs layer of the DBR structure 12 (i.e.,
bottom layer of the structure 12 bonded to the spacer), which acts
as an etch-stop layer and which is also selectively etched in a
HF--H.sub.2O solution.
[0035] In the third step, a mesa is etched in the DBR 12 by dry
etching in Cl.sub.2--CH.sub.4--Ar and selective chemical etching in
a HF--H.sub.2O solution till reaching the etch stop-layer 19 to
form the top DBR stack 12a (FIG. 1), which is centered around a
vertical axis passing through the center of the recess 16 and has
the lateral dimension of 80.times.80 .mu.m.sup.2. As a result of
this etching, the membrane 23 is formed as the lateral continuation
of the supporting structure 18 (its region 18b) completely covering
the recess 16. By this, the air-gap cavity 14 is formed being
confined at its bottom side by the top surface of the bottom DBR
stack 12b and at its top side by the supporting structure 18. The
device fabrication is completed by forming the electrical contacts
26.
[0036] In the present example, the spacer structure 17 and the
supporting structure 18 are made of pairs of GaAs/AlGaAs layers. It
should, however, be noted that these structures, as well as those
of the DBR stacks, can also be made of GaAs, or other types of
dielectric layers. In order to stabilize the transmitted optical
mode, a mesa can be formed on the bottom of the recess 16 being
centered around the vertical axis passing through the center of the
recess and having the lateral size of less than 10 and height of
less than {fraction (1/30)} of the device operation wavelength.
[0037] Referring to FIG. 3, there is illustrated a tunable vertical
cavity device 100 according to another embodiment of the present
invention presenting a VCSEL device structure. This device is
designed to emit light in the vicinity of 1.55 .mu.m. To facilitate
understanding, the same reference numbers are used for identifying
those components, which are identical in the devices 10 and 100.
Similar to the device 10 of the previous example, the device 100 is
designed like a tunable Fabry-Perot cavity having top and bottom
DBRs 12a and 12b, respectively, with maximum reflectivity at 1.55
.mu.m. In distinction to the previously described device 10, in the
device 100, the spacer 17 is placed on the top of an active cavity
material 27, which is fused to the surface of the AlGaAs/GaAs
bottom DBR stack 12b.
[0038] The active cavity material 27 comprises a multiquantum well
InGaAsP/InGaAs layer stack 28, which has a maximum of
photoluminescence emission at 1.55 .mu.m and is sandwiched between
two InP cladding layers 29 and 34. The optical thickness of the
active cavity material is equal to 3/2.times.1.55 .mu.m. The spacer
17 has a total thickness of 1.51 .mu.m and comprises a InP layer 30
with alternating p-n-p-n doping sandwiched between 2 InGaAsP
etch-stop layers 31 and 32. The spacer 17 is grown in the same
process with the active cavity material 27. A mesa 33 made of
InGaAsP and having the maximum of photoluminescence (PL.sub.max) at
1.41 .mu.m is located on the bottom of the recess 16 and centered
about a central vertical axis passing through the center of the
recess 16.
[0039] The device 100 may be pumped optically with 980 nm pump
light, for example, through the top DBR 12a, resulting in an
emission at 1.55 .mu.m through the bottom DBR 12b and the GaAs
substrate 11. Applying a voltage between contacts 26 results in a
deflection of the membrane 23 towards the bottom of the recess 16,
which shortens the air-gap cavity 14 and correspondingly, the
emission wavelength of the VCSEL device as well. The mesa 33
introduces a lateral refractive index variation in the optical
cavity allowing to stabilize the optical mode. The height and the
lateral size of the mesa 33 should be set less than {fraction
(1/30)} and less than 10, respectively, of the device operation
wavelength.
[0040] The fabrication of the tunable VCSEL device 100 will now be
described with reference to FIGS. 4 and 5.
[0041] First, a multilayer stack structure 40 is grown on a InP
substrate 35. The structure 40 comprises the spacer 17 and the
active cavity material 27. The spacer 17 has the total thickness of
1.5 .mu.m and includes an InP layer 30 with alternating p-n-p-n
doping sandwiched between two etch stop InGaAsAP layers, both with
PL.sub.max=1.4 .mu.m and thickness of 50 nm. The active cavity
material 27 has the total thickness of 725 nm and comprises 6
quantum wells sandwiched between two InP cladding layers.
[0042] Then, the fusion of the multilayer stack 40 with the bottom
DBR stack 12b is performed by putting them face to face in a
forming gas ambient, increasing the temperature to 650.degree. C.,
and applying a pressure of about 2 bar to the fused interface. This
process is followed by selective etching of the InP substrate 35 in
a HCl--H.sub.2O solution till reaching the InGaAsP etch-stop layer
32 to form the recess 16. More specifically, the selective etching
consists of the following: The InGaAsP etch-stop layer 32 is first
etched in an H.sub.2SO.sub.4--H.sub.2- O.sub.2--H.sub.2O solution,
and then the InP layer 30 is etched in a HCl--H.sub.2O solution.
Thereafter, the mesa 33 is formed by etching in a
H.sub.2SO.sub.4--H.sub.2O.sub.2--H.sub.2O solution.
[0043] In the next step, the structured surface of the spacer 17 is
fused to the substantially planar surface of the supporting
structure 18. The fusion is performed at 650.degree. C. applying a
pressure of 2 bar to the fused interface. This is followed by
selective etching of the GaAs substrate 25 of the top DBR wafer 24,
and by etching the DBR 12 as described above with respect to the
fabrication of the device 10 to form the mesa 12a. The device
fabrication is completed by forming the electrical contacts 26.
[0044] Those skilled in the art will readily appreciate that
various modifications and changes can be applied, to the preferred
embodiment of the invention as hereinbefore exemplified without
departing from its scope defined in and by the appended claims.
Note that the publications designated with an astrick* are
incorporated herein by reference thereto.
* * * * *